Multiplexed radio communication system

ABSTRACT

A multiple access communications system reuses a set of either N carrier frequencies or time slots in adjacent communications sites to provide more than N or T minimally cross-correlated frequency or time hopping communications channels respectively. A first set of communications channels is associated with a first of the communications sites. No two of the channels in the first set employ the same frequency or T time slots at the same time. A second set of communications channels is associated with a second of the adjacent communications sites. No two of the channels in the second set of channels employ the same one of the N carrier frequencies or T time slots at the same time. One or more sets of the minimally cross-correlated channels are further defined so that none of the channels in such sets employ the same frequency at the same time as more than a predetermined number of the channels in another of the sets of the minimally cross-correlated frequency-hopping communications channels. This is accomplished by having such other sets be a decimated transformation of the first or second set respectively.

APPLICATION DATA

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/080,075 filed Jun. 18, 1993, now issued as U.S. Pat. No.5,408,496, on Apr. 18, 1995 and it is also a continuation-in-part ofU.S. application Ser. No. 08/178,887 filed Jan. 7, 1994, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to radiotelephone systems, and, moreparticularly, relates to methods and apparatus for implementingmultiplexing techniques in a radiotelephone communications system foruse in special mobile radio (SMR).

A plurality of communications channels may be defined in a givenbandwidth of the radio frequency spectrum to provide a radiotelephonesystem by assigning a plurality of distinct carrier frequencies in thebandwidth to define each channel. Such systems are calledfrequency-division multiple access (FDMA) systems. Alternatively, thecommunications channels may be defined by assigning discrete time slotsfor using a given carrier frequency. Such systems are called timedivision multiple access (TDMA) systems. In a still different system thechannels may be defined by what is known as code division multipleaccess (CDMA).

One type of communications system that Can be a CDMA system is a spreadspectrum system. Spread spectrum communications systems can beimplemented as multiple access systems in a number of differentconfigurations. One type of multiple access spread spectrum system is acode division multiple access (CDMA) system. CDMA spread spectrumsystems may use direct sequence (DS-CDMA) or frequency hoping (FH-CDMA)spectrum spreading techniques. FH-CDMA systems can be further dividedinto slow frequency hopping (SFH-CDMA) and fast frequency hopping(FFH-CDMA) systems. In SFH-CDMA systems, several data symbols,representing a sequence of data bits that are to be transmitted,modulate the carrier frequency within a single hop. In FFH-CDMA systems,in contrast, the carrier frequency hops (changes) several times per datasymbol.

FH-CDMA techniques have been proposed for cellular radiotelephonesystems by Cooper and Nettleton. DS-CDMA was proposed in the context ofcellular systems by Gilhousen et al.

There is increased channel capacity in a CDMA system over an FDMAsystem. The reason is that even though both types of systems areinterference limited, the capacity of a FDMA system is determined by thelargest interference that may exist in the bandwidth, whereas thecapacity of a CDMA system is determined by the average interference overthe entire bandwidth. Such average interference is usually much smallerthan the worst case interference, unless the interference is the same inall parts of the bandwidth. Additionally, CDMA systems inherentlyincorporate frequency diversity, which mitigates multipath effects.Further, because of the interference averaging ability of the CDMAsystem, the employment of Voice Activity Detection and DiscontinuedTransmissions (VAD) techniques increases the capacity by reducing theaverage interference level by the duty ratio of the speech. By utilizingappropriate parameters, both DS-CDMA and FH-CDMA can provide similaraveraging capabilities.

A further advantage of FH-CDMA systems is that the bandwidth employedneed not be contiguous.

Frequency hopping and direct sequence techniques have been proposed andutilized in a number of spread-spectrum radio-telephone systems.Examples of such systems are set forth in the following:

    ______________________________________                                        3,239,761           Goode                                                     5,048,057           Saleh et al                                               4,066,964           Costanza et al                                            4,176,316           DeRosa et al                                              4,554,668           Deman et al                                               4,979,170           Gilhousen et al                                           5,099,495           Mikoshiba et al                                           4,901,307           Gilhousen et al                                           5,051,998           Murai et al                                               4,222,115           Cooper et al                                              4,704,734           Menich et al                                              4,933,954           Petry                                                     5,010,553           Scheller et al                                            5,065,449           Gordon et al                                              5,067,173           Gordon et al                                              4,144,411           Frenkiel                                                  4,794,635           Hess                                                      5,056,109           Gilhousen et al                                           5,179,569           Sawyer                                                    EP 391,597                                                                    UK 2,242,806                                                                  W091/13502                                                                    W091/15071                                                                    W091/12681                                                                    W091/12681                                                                    U.K. Patent Application 2,242,806                                             U.K. Patent Application 2,242,805                                             ______________________________________                                    

Cooper et al, "A SPREAD SPECTRUM TECHNIQUE FOR HIGH CAPACITY MOBILCOMMUNICATIONS". 1978, IEEE

Viterbi, "NON LINEAR ESTIMATION OF PSK-MODULATED CARRIER PHASE WITHAPPLICATION TO BURST DIGITAL TRANSMISSION". 1982, IEEE

Omura et al, "CODED ERROR PROBABILITY EVALUATION FOR ANTI JAMCOMMUNICATION SYSTEMS". 1982, IEEE

Lempel et al, "FAMILIES OF SEQUENCES WITH OPTIMAL HAMMING CORRELATIONPROPERTIES". 1973, IEEE

Verhulst et al, "SLOW FREQUENCY HOPPING MULTIPLE ACCESS FOR DIGITALCELLULAR RADIO TELEPHONE". 1984, IEEE

Mathematics which can be used for achieving orthogonality in a FH-CDMAsystem was suggested by H. Greenberger in an article "Families ofSequences with Optimal Hamming Correlation Properties" published in IEEETransactions on Information Theory, Vol. IT 20, No. 1 Jan. 1974.

U.S. Pat. No. 4,850,036 to Smith is directed to a dialing andsynchronization sequence for a frequency hopping radiotelephonecommunication system. This patent teaches a system in which allfrequency hopping channels are defined by using a sequence of carrierfrequencies within a bandwidth such that no one carrier frequency isused by more than one channel at the same time. In this system, fewerfrequency hopping channels can be attained in a given bandwidth thanwould be provided if each carrier frequency defined a separate channel.

U.S. Pat. No. 4,554,668 to Deman et al. discloses a frequency hoppingradio communications system in which a master station is used tocommunicate digitally with a plurality of slave stations. Each slavestation has a fixed carrier frequency sequence, permanently assigned toit, to define its communications channel. Timing information isextracted from the data stream.

UK patent application GB 242 805 A of Ramsdale et. al. discloses thatinterference can be reduced if a cell is seotorized into a group ofsmaller cells by means of a directional antenna; and also discloses thatfor reasons of interference reduction, adjacent microcells normally useddifferent channels, as determined by a channel allocation scheme.However, when movement of a handset is detected (such as by marginallyBER, low field strength or delay measurements), then a common "umbrella"channel is allocated to that handset in all of the microcells within agroup of adjacent of nearby cells, that is a sub-array of the array.

U.S. Pat. No. 4,901,307 to Gilhousen indicates that in order to obtain alarge number of users they use forward error correcting codedcommunication signals using code division multiple access (CDMA) spreadspectrum transmission, and discloses the use of different size cells.This patent also discloses beam steering with a directional antenna toreduce interference in a CDMA spread spectrum radio telephone system,and a phase array antenna.

Application WO 92/00639 discloses that information communicated on thecellular-to-mobile link channels are encoded, interleaved, bi-phase(BPSK) modulated with orthogonal covering of each BPSK symbol along withquadrature phase shift key (QPSK) spreading of the covered symbols.

An article entitled Slow Frequency Hopping Multiple Access for DigitalCellular Radiotelephone by Verhulst et. al, published in IEEE Journal onSelected Areas in Communications, Vol. Sac-2, No. 4, July 1984, page563, discloses that one drawback of frequency hopping multiple access isa reduction of spectrum efficiency, but that if power control andsilence detection are used, good capacity can be attained.

U.S. Pat. No. 4,144,411 to Frenkiel discloses the use of different cellsizes in a mobile communications system.

PCT application WO 91/15071 discloses the use of a multiplicity of cellsreferred to as clusters.

U.S. Pat. No. 4,704,734 discloses a Method and Apparatus for SignalStrength Measurement and Antenna Selection in Cellular Radio TelephoneSystems.

European Patent Application EP 0 189 695 discloses a frequency hoppingCDMA system having a mixed allocation of sequence protocol which waspublished in the European Patent Bulletin on Sep. 20, 1989 and inEnglish on Jan. 17, 1990.

PCT application WO 91/12681 discloses an Interconnecting and ProcessingSystem for Facilitating Frequency Hopping.

PCT application WO 91/13502 discloses a system utilizing Shared-CarrierFrequency-Hopping.

U.S. Pat. No. 5,056,109 discloses a power control system that acts inresponse to power in the communications signal received and signals thatare generated at the remote station that are transmitted back.

U.S. Pat. No. 5,048,057 to Saleh et al. discloses a Wireless Local AreaNetwork utilizing codes exhibiting built-in diversity, and the use ofside information by the decoder to improve its ability to accuratelyrecover data in the presence of interference. This patent also mentionssoft decision decoding.

PCT patent application number WO 92/00639 discloses a system with pathdiversity for a local area wireless telephone system.

Analagous hopping techniques may also be utilized in the time domain. Insuch systems, a single carrier frequency is divided into time slots asis done in a TDMA system. However instead of assigning a fixed time slotto provide a communications channel, time slots are assigned in apredetermined sequence to provide a time hopping communications channel.

U.S. Pat. No. 5,291,455 to Bruckert discloses one type of time hoppingsystem which is a hybrid of time and frequency hopping wherein givenchannels of a TDMA system are defined by a series of different timeslots on particular frequencies. It does not disclose the use oforthogonal sets of time hopping channels in adjacent sites nor does itdisclose the use of minimally cross correlated sets of hopping channels.As used herein, with respect to both frequency hopping and time hoppingsystems, the terminology "minimally cross correlated" means that no oneof the channels in such set or sets of the minimally cross correlatedcommunications channels employ the same one of the N hops concurrentlyas more than a predetermined number of the channels in another of thesets of the minimally cross correlated communication channels.

Both frequency hopping systems, time hopping systems and time-frequencyhopping systems may be generically referred to as hopping systems. Suchhopping systems, however, have a number of deficiencies. In particular,in some such systems, in order to define channels with minimuminterference, the number of usable communications channels defined isless than the number of discrete, carrier frequencies used in the caseof frequency hopping, and less than the number of slots per time framein a time hopping system. This is the characteristic, for example, ofthe system set forth in the Smith patent listed above.

It is an object of this invention to provide an improved hoppingradiotelephone system for use in SMR systems.

It is accordingly an object of the invention to provide improvedradiotelephone communication methods and apparatus.

It is another object of the invention to provide a hoppingradiotelephone communication system wherein the number of discrete,usable communications channels exceeds the product of the number ofassigned carrier frequencies multiplied by the number of time slots on asingle carrier frequency. Similarly, it is an object of the invention toprovide an improved time hopping radiotelephone communication systemwherein the number of discrete, usable communications channels exceedsthe number of time slots per frame.

It is a further object of the invention to provide a radio-telephonecommunications system such that interference is more evenly distributedamong the communications channels to provide more quality communicationschannels.

Other general and specific objects of the invention will in part beobvious and will in part appear hereinafter.

SUMMARY OF THE INVENTION

The foregoing objects are attained by the present invention, whichpertains to either frequency hopping, time hopping or hybrid hoppingcommunication systems. For ease of reference herein such systems shallbe generally referred to as hopping systems and the communicationschannels utilized shall be referred to as hopping channels with theunderstanding that this terminology refers to hopping in either or boththe frequency domain and/or the time domain. The term "communicationsresource" means a carrier frequency for frequency hopping and a timeslot on a carrier for time hopping and time-frequency hopping.

The invention provides in one aspect a multiple access communicationssystem in which a set of R communications resources are reused inadjacent communications sites to provide greater than R hoppingcommunications channels. The system includes a method and apparatuswhich defines a first set of hopping communications channels associatedwith a first of the communications sites in which no two of the channelsin the first set employ the same one of the R communications resourcesat the same time. The system further includes a method and apparatuswhich further defines a second set of the hopping communicationschannels associated with a second of the adjacent communications sitesin which no two of the channels in the second set employ the same one ofthe R communications resources at the same time.

In a further aspect of this invention the system also includes a methodand apparatus which defines a third set of hopping communicationschannels associated with a third of the adjacent communications sites inwhich no two of the channels in the third set employ the same one of theR communications resources at the same time.

Yet another aspect of this invention is that the second and third setsof the hopping communications channels are decimated transformations ofeach of the hopping communications channels in the first set.

Each of the hopping communications channels in the first set is definedby a unique sequence of the frequencies or time slots and the firstdecimating transformation is performed on each of the hoppingcommunications channels in the first set by selecting slots from each ofthe hopping communications channels in the first set in their sequentialorder skipping a first decimation number of slots in the sequence andrepeating this process on the remaining slots in the sequence of eachchannel in their remaining order until all of the slots in each channelare used to define a second set of hopping communications channels.Further, a second decimating transformation is performed on each of thehopping communications channels in the first set by selecting slots fromeach of the hopping communications channels in the first set in theirsequential order skipping a second decimation number of slots in thesequence and repeating this process on the remaining slots in thesequence of each channel in their remaining order until all of the slotsin each channel are used to define a third set of hopping communicationschannels, wherein the first and second decimation numbers are differentand each is less than the minimum factor of N.

In a yet further aspect of this invention the system also includesapparatus for selectively encoding digital information signals oncertain of the hopping communications channels so that there is aredundant relationship between channel bits. The error correcting codesets this relationship and the decoder utilizes it for error correction.

In the preferred embodiment of this invention soft decision making andside information are employed in decoding. Voice activity detection isalso employed to measure signal activity levels for selectivelyallocating channels to subscribers. Apparatus is also provided forperforming a conference call between a plurality of subscribers in asite by causing each of the subscribers to employ the same channel.Additional subscribers can be included in such conference call in othersites by using the same approach and other conventional conferencingmethods can be used to add parties using other telephone systems.

The invention further contemplates an electronically controlled antenna,apparatus responsive to a control signal for controlling the antenna toprovide a first antenna radiation pattern for defining the first of theadjacent communications sites, apparatus responsive to a control signalfor controlling the antenna to provide a second antenna radiationpattern which overlaps with the first antenna radiation pattern at aboundary for defining the second of the adjacent communications sites;and apparatus responsive to the number of the system subscribers usingeach of the first and second of the adjacent communications sites foraffecting the control signal to move the boundary.

The invention also contemplates apparatus for defining a firstmicro-site within one or more of the adjacent communications sites, thefirst micro-site also reusing the R communications resources. Themicro-site has no greater than 10% the average power as the adjacentcommunications site it is in.

The present invention contemplates the option of direct communicationbetween subscribers if they are both in the micro-site by causing one ofthe two subscribers to communicate uplink with a first downlink channeland downlink with an first uplink channel, and causing the other of thetwo subscribers to communicate uplink on the first uplink channel anddownlink on the first downlink channel.

The invention will next be described in connection with certainillustrated embodiments; however, it should be clear to those skilled inthe art that various modifications, additions and subtractions can bemade without departing from the spirit or scope of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the invention,reference should be made to the following detailed description and theaccompanying drawings, in which:

FIG. 1 is a schematic diagram depicting a first plurality ofcommunications sites operating in accordance with the mobilecommunications system of the invention particularly suited for SMRsystems;

FIG. 2 is a schematic diagram depicting operation of another embodimentof the invention, and showing a second plurality of communications sitessuited for cellular systems;

FIGS. 3A, 3B, 3C depict the construction of communications channels fromsequences of frequencies in accordance with the invention;

FIG. 4 depicts a decimation transform in accordance with the invention;

FIG. 5 depicts the relationship between hops and transmission of digitalinformation in accordance with the invention;

FIG. 6 depicts the construction of communications channels fromsequences of time slots in accordance with the invention;

FIG. 7 depicts the construction of communications channels for adjacentsectors or cells from sequences of time slots in accordance with theinvention; and

FIG. 8 is a block diagram depicting an encoding/decoding configurationin accordance with the invention;

FIG. 9 is a block diagram depicting the structure of a voice activitydetection circuit in accordance with the invention;

FIG. 10 is a block diagram depicting duplex operation in accordance withthe invention; and

FIG. 11 is a block diagram depicting an embodiment of the inventionutilizing an electronic antenna and boundary control apparatus.

DESCRIPTION OF ILLUSTRATED EMBODIMENTS Introduction

Set forth below are separate descriptions of both frequency hopping andtime hopping embodiments of the present invention. Both types of systemscan be used in conjunction with the sectorized and cellular sites asdepicted in FIGS. 1 and 2. Further, the decimation transform of FIG. 4and the depiction of hopping in FIG. 5 apply to both types of systems.The same principles also apply to time-frequency hopping systems.

Frequency Hopping System

Referring to FIG. 1, a geographic service coverage area scheme is shown.The geographic area of coverage 100 is divided into a plurality ofcommunications sites 102, 104, 106, 108 which are each served by acentral base communications station 110 having a sectorized antenna 112.Of course as in most real systems there is not perfect geographicisolation between the various sectors. One sector of the sectorizedantenna 112 defines each of the sites 102, 104, 106, and 108.

In a frequency hopping system, a set of N carrier frequencies are reusedin adjacent communications sites to provide greater than N frequencyhopping communications channels. This result is attained by defining afirst set of frequency hopping communications channels associated withthe first of the communications sites 102 such that no two of thechannels in the first set employ the same one of the N carrierfrequencies at the same time. Apparatus and methods for defining thefirst set of communications channels is discussed in greater detailhereinafter in connection with FIGS. 3 and 4.

A second set of the minimally cross correlated frequency hoppingcommunications channels associated with the second of the adjacentcommunications sites 104 is defined such that no two of the channels inthe second set employ the same one of the N carrier frequencies at thesame time. Apparatus and methods for defining the second set ofcommunications channels is discussed in greater detail hereinafter.

Moreover, a third set of frequency hopping communications channelsassociated with the third of the adjacent communications sites 106 isdefined such that no two of the channels in the third set employ thesame one of the N carrier frequencies at the same time. Apparatus andmethods for defining the third set of communications channels isdiscussed in greater detail hereinafter.

In a preferred embodiment of the invention, at least one set of thefrequency hopping channels is defined so that no one of the channels insuch set or sets of the employ the same one of the N carrier frequenciesat the same time as more than a predetermined number of the channels inanother of the sets. This predetermined number of channels is theminimum number of channels possible. In the preferred embodiment thepredetermined number is one. This property is discussed in greaterdetail hereinafter in connection with FIGS. 3A, 3B, 3C, and 4.

Conventional CDMA systems can operate only under a multi-cell frequencyreuse pattern, which is required to control interference. This may causeserious frequency management problems, particularly when cells are addedto an existing cellular system. On the other hand, the CDMA systemdescribed herein can implement a one cell frequency reuse pattern--i.e.,the same frequencies can be reused in every communications site--thusmitigating the frequency planning problem that hampers current cellularsystems. Furthermore, a cell may be divided into more than onecommunication site, as depicted in FIG. 1, which can be an importantsource for capacity increase. For example, by dividing each omni-cellinto four communications sites, as indicated in FIG. 1, each using thesame N carrier frequencies, significant additional channel capacity canbe attained in the geographic area served, as compared with a systemthat does not have the same frequency reuse pattern.

FIG. 2 is a schematic diagram depicting operation of another embodimentof the invention having a plurality of communications sites suited forcellular systems. In the illustrated cellular configuration, thegeographic area of coverage 200 is divided into four communicationscells 202, 204, 206, 208 which are each served by a central basecommunications station and corresponding antenna 212, 214, 216, 218. Asin the system discussed in connection with FIG. 1, perfect geographicisolation does not exist between the various cells. In particular, areasof overlap exist between the communications cells 202, 204, 206, 208. Inconventional cellular systems, interference in these areas of overlaphas posed significant difficulty. In connection with the invention,however, interference in the regions of overlap is minimized in themanner described above with regard to FIG. 1. More specifically, thesystem provides sets of self orthogonal frequency hopping communicationschannels, wherein such sets are characterized by minimal crosscorrelation between channels of different sets. When implemented in acellular configuration, as illustrated in FIG. 2, the FH-CDMA systemdescribed herein yields a one cell reuse pattern. These aspects arediscussed in greater detail hereinafter.

FIGS. 3A, 3B, 3C depict the construction of communications channels fromsequences of carrier frequencies in accordance with the invention. Inparticular, the minimally cross correlated frequency hoppingcommunications channels described above are defined in accordance withthe code division technique illustrated in FIGS. 3A, 3B, and 3C.

FIG. 3A is a chart relating each channel 1, 2, 3, 4, to a unique seriesof frequency-hopping sequences, thereby indicating the manner in whichfour orthogonal communications channels 1, 2, 3, 4 are defined from theset of N frequencies 1, 2, 3, 4, . . . , N. It should be noted thatsequences 1, 2, 3, and 4 are identical to each other but are eachshifted one time slot from each other, such that sequences 1, 2, 3, and4 are mutually orthogonal.

As illustrated in FIGS. 3A, 3B, and 3C, multiple communication channelsusing the same carrier frequencies are attained by allocating thecarrier frequencies to each communications channels at preselectedtimes. A plurality of hopping sequences are used to assign the carrierfrequencies to different channels during the time periods. These uniquehopping sequences are selected so that they are orthogonal to oneanother in each site or sector, such that the cross-correlation betweenthe hopping sequences for a given site or sector is zero.

Particular transmitted signals can be retrieved from the communicationschannel defined by such a hopping sequence by using the hopping sequencein the receiver.

The hopping sequences are selected such that users in each site areassigned mutually orthogonal sequences, and inter-site correlation offrequency-hopping sequences is theoretically zero. In the preferredembodiment of this invention there is only one time that any carrierfrequency when associated with a sequence defining a particular channelin one site interferes with any particular channel in adjacent sites.Known Forward Error Correction (FEC) and interleaving techniques can beemployed in the system described herein to mitigate remaininginterference. A system architecture providing these features isillustrated in FIG. 8.

With proper selection of system parameters, the frequency-hopping codedivision multiple access (FH-CDMA) system described herein offersadvantages previously asserted for direct sequence (DS-CDMA) systems. Inaddition, the user capacity of FH-CDMA is enhanced by the intrinsicinterference averaging afforded by the system, and can readily exploitthe intermittent duty cycle associated with voice activity. Moreover,due to the orthogonal operation described herein, interference fromco-users of a user's site is eliminated. Since this is the major sourceof interference in non-orthogonal systems typical of DS-CDMA systems,FH-CDMA yields higher capacity and enhanced performance capabilitieswhen structured as set forth above. When implemented in a cellularconfiguration, as illustrated in FIG. 2, the FHCDMA system describedherein also yields a one site or cell reuse pattern.

From an implementation standpoint, the FH-CDMA system described hereincan be readily implemented with existing technologies. In particular,the mobile power control problem fundamental in DS-CDMA is much reducedin FH-CDMA. The one cell frequency reuse pattern alleviates thefrequency management problem, which exists in current cellular systems.

Yet another aspect of this invention is that the second and third setsof the minimally cross correlated frequency hopping communicationschannels are decimated transformations of each of the minimally crosscorrelated frequency hopping communications channels in the first set.

FIG. 4 depicts a decimation transform utilized in one practice of theinvention. In accordance with the transform depicted in FIG. 4, each ofthe frequency hopping communications channels in the first set isdefined by a unique sequence of the frequencies and the decimatingtransformation is performed on each of the channels in the first set byselecting frequencies from each of the channels in the first set intheir sequential order, skipping a first decimation number offrequencies in the sequence, and repeating this process on the remainingfrequencies in the sequence of each channel in their remaining order,until all of the frequencies in each channel of the first set are usedto define a second set of frequency hopping communication channels whichare minimally cross correlated with respect to the first set.

A second decimating transformation is performed on each of the frequencyhopping communications channels in the first set by selectingfrequencies from each of the channels in the first set in theirsequential order, skipping a second decimation number of frequencies inthe sequence, and repeating this process on the remaining frequencies inthe sequence of each channel in their remaining order, until all of thefrequencies in each channel of the first set are used to define a thirdset of frequency hopping channels which are minimally cross correlatedwith respect to the first and second sets.

In accordance with the invention, the first and second decimationnumbers are different and each is less than the minimum factor of N,where the minimum factor of a number is the smallest number, greaterthan one, that can be divided into the number with a remainder of zero.

For example, suppose that Channel 1 of a first set of orthogonalfrequency hopping communications channels associated with a firstcommunications site is defined by the following frequency hoppingsequence:

    1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 . . .                        (Set 1, Ch.1)

wherein the numbers 1, 2, 3, 4, and 5 represent discrete carrierfrequencies. Decimation is executed on Channel 1 of the first set byselecting frequencies from the above-listed frequency hopping sequencein sequential order, skipping a first number of frequencies in thesequence. This number is referred to herein as the "decimation number"or "decimation factor." For purposes of this example, suppose adecimation factor of 3. Decimating the above sequence with a decimationfactor of 3 yields the following frequency hopping sequence:

    4 3 2 1 5 4 3 2 1 5 4 3 2 1 5 . . .                        (Set 2, Ch.1)

In accordance with the invention, this sequence is used to defineChannel I of the second set of channels, which is associated with asecond communications site.

Frequency hopping sequences for the remaining channels of Set 2 areconstructed similarly, by decimating the sequences for each of thechannels of Set 1 by the same decimation factor of 3 such that set 1 andset 2 are minimally cross correlated with respect to one another.

Then, to generate the frequency hopping sequence defining Channel 1 ofthe third set of channels--which is associated with the thirdcommunications site--the Set 1, Channel 1 sequence is decimated by adifferent decimation number, selecting frequencies from the Set 1,Channel 1 sequence in their sequential order, skipping the seconddecimation number of frequencies in the sequence. In particular, thefollowing sequence can be decimated, for example, by the decimationfactor of 2, such that the decimation of the sequence

    ______________________________________                                                 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 . . .                                                               (Set 1, Ch. 1)                                   yields                                                                                 3 1 4 2 5 3 1 4 2 5 3 1 4 2 5 . . .                                                               (Set 3, Ch. 1)                                   ______________________________________                                    

This process is repeated on the remaining frequencies in the sequence ofeach channel in their remaining order until all of the frequencies ineach channel are used to define a third set of frequency hoppingcommunications channels which are minimally cross correlated withrespect to the first and second sets.

It will be appreciated that the above-described operations provide setsof sequences having minimum cross-correlation, and supportcommunications sites or sectors in which no two of channels in a givensite employ the same one of the allocated frequencies at the same time.The resulting system is thus characterized by orthogonal operationwithin a given site or sector, and minimum sector-to-sectorcross-correlation.

This system has the property that the number of available adjacentcommunications sites is one less than the minimum factor of the numberof assigned frequencies. The minimum factor of a number is the smallestnumber, greater than one, that can be divided into the number with aremainder of zero. Thus, if the number of assigned frequencies is three,then the minimum factor is three, and the maximum number of adjacentminimally cross correlated communications sites is two. If the number ofassigned frequencies is four, then the minimum factor is two, and onlyone site can be serviced.

Similarly, if the number of frequencies utilized is 10, then only onesite can be serviced. If the number of frequencies utilized is seven,then six sites can be serviced. Accordingly, the maximum number of sitescan be serviced by a system of this invention, using a given number N offrequencies, if N is a prime number. Thus, in a preferred embodiment ofthis invention, the number N of frequencies is a prime number.

The capacity of the systems described herein is determined by theiraverage interference. Such average interference is usually much smallerthan the worst case interference, unless the interference is uniform. Ina multiple-site embodiment of the invention, interference can occur inthe form of collisions, i.e., simultaneous use of the same frequency byusers in the same communications site, or by users in a differentcommunications site. System performance depends upon the probability ofcollisions, and the power of the colliding interferer.

In order to reduce the effects of such collisions, a mobilecommunications system in accordance with the invention encodes andinterleaves digital information signals on the minimally crosscorrelated, frequency hopping communications channels such that severalinformation-representative digital channel symbols are transmittedduring each hop. As depicted in FIG. 5, in accordance with the slowfrequency hopping scheme employed by the invention, one hop occurs forevery M channel symbols, where, for example, M=6.

Accordingly, as indicated in FIG. 5, corrupted bits from an interferedhop are separated by a number of channel symbols S. This redundancyenables the utilization of known digital error correction techniques,such as Forward Error Correction (FEC), which detects errors due tocollisions, rejecting selected ones of the redundant bits as INVALID,and accepting others as VALID. In accordance with the invention, atleast two techniques can be implemented to determine which ones of theredundant bits are to be accepted as VALID.

Under the first technique, a complex measure of each sampled value ofthe received signal is projected on a decision chart and is assigned avalue (metric) that corresponds to the decision regions of such chartthat include the sampled value. The metric is fed to a conventional FECdecoder for soft error correction and decision. An example of aconfiguration utilizing an FEC decoder is depicted in FIG. 8.

Under the second technique, a carrier/interference (C/I) ratio isestimated for each hop. This estimate is fed to the FEC decoder toimprove its performance. This technique can be implemented by detectingeach hop whose C/I ratio is below a predetermined threshold, and foreach such hop, replacing the metric corresponding to such hop with anull metric that does not affect the decision process of the FECdecoder.

In the embodiment depicted in FIG. 8, system performance is furtherenhanced by implementing in the decoder the known techniques of softdecision making and the use of side information.

In a preferred embodiment of the invention, system performance is alsoimproved by employing Voice Activity Detection (VAD), which increasessystem capacity by reducing the average interference level by the dutyratio of the speech.

FIG. 9 is a block diagram depicting the structure of a voice activitydetection circuit in accordance with the invention. The circuit 600includes a voice activity detection module 602, a transmission controlmodule 604 and a channel allocation control module 606. The voiceactivity detection module 602 measures signal activity levelsrepresentative of subscriber voice activity on an assigned channel togenerate a channel voice activity signal. A preferred embodiment of theinvention includes elements responsive to the voice activity signal forselectively allocating channels to subscribers. A voice activity signalindicative of silence is used in a decision making program to deactivatethe channel in question through the transmission control module 604. Asecond program reallocates a channel to the subscriber upon resumptionof voice activity through the channel allocation module 606.

A preferred embodiment of the invention provides apparatus for definingat least a first micro-site within one or more of the adjacentcommunications sites, as depicted in FIG. 1. Each micro-site reuses thesame N frequencies as are used by the adjacent communications sites. Themicro-site is characterized, for example, by an average power of lessthan or equal to 10% the average power as the adjacent communicationssite in which it is situated. This power ratio is provided by way ofexample only, and other micro-site power levels may be employed inconnection with the invention. A micro-site is utilized for high usageareas, or areas in electromagnetic shadows, or to extend acommunications sector or site.

Under current governmental regulations, two sets of frequencies areassigned by the FCC for SMR services. One set is utilized for uplinktransmission from mobiles to the base station, while the other isdedicated to downlink transmission from the base station to mobileunits. A fixed gap, for example, 39 MHz or 45 MHz is maintained betweenthese two sets, and channels are assigned in pairs, one for downlink andone for uplink.

The present invention utilizes appropriate allocation of downlink anduplink channels to provide the option of direct communication betweensubscribers who are in the same micro-site. Such communication isreferred to as duplex operation. FIG. 10 is a block diagram depictingduplex operation in accordance with the invention. As illustrated inFIG. 10, the subscribers can be directly linked by allocation of uplinkand downlink channels. In particular, duplex operation is implemented bycausing one of the two subscribers to communicate uplink with a firstdownlink channel and downlink with a first uplink channel, and causingthe other of the two subscribers to communicate uplink on the firstuplink channel and downlink on the first downlink channel.

Thus, in accordance with the invention, the capability of full duplexoperation is provided so that two subscribers can communicate directlyto each other without going through the base station, by having onesubscriber assigned the same uplink channel as the other subscriber withwhom direct communication is to be achieved has for its downlink channeland vice versa. Similarly, the configuration depicted in FIG. 8 providesthe capability for performing a conference call between multiplesubscribers within a given communications site by causing each of saidsubscribers to employ the same channel.

As indicated by dashed lines in FIG. 1, the boundaries of thecommunications sites are flexible. In this regard, a preferredembodiment of the invention, depicted in FIG. 11, employs anelectronically controlled antenna, apparatus responsive to a controlsignal for controlling the antenna to provide a first antenna radiationpattern for defining the first of the adjacent communications sites,apparatus responsive to a control signal for controlling the antenna toprovide a second antenna radiation pattern which overlaps with the firstantenna radiation pattern at a boundary for defining the second of theadjacent communications sites; and apparatus responsive to the number ofsystem subscribers using each of the first and second of adjacentcommunications sites for causing the control signal to move theboundary.

Referring again to FIG. 2, the cellular system depicted therein utilizesfour antennas having respective antenna radiation patterns for definingfirst, second, third and fourth adjacent communications sites. A fifthantenna utilized in the system has a respective fifth antenna radiationpattern for defining a micro-site within the first communications site.The micro-site reuses the same N frequencies utilized by the pluralityof adjacent communications sites, while the radiation pattern of thefifth antenna defining the micro-site may be characterized, for example,by a mean power of no more than ten percent that of the other antennaradiation patterns. The ten percent power level is provided by way ofexample only, and other power levels may be utilized to define amicro-site.

It will thus be seen that the invention efficiently attains the objectsset forth above, among those made apparent from the precedingdescription. In particular, the invention provides a mobilecommunications system in which the number of usable communicationschannels exceeds the number of allocated carrier frequencies. TheFH-CDMA system described herein provides a capacity that is superior toall known cellular systems.

TIME-HOPPING EMBODIMENT

The invention will be now described with respect to a time hoppingembodiment. FIG. 6 depicts a series of five time frames supported by acarrier frequency. The time frames are divided into five time slots(S1-S5) each. A plurality of 15 slot selection sequences for utilizingthe carrier frequency for 15 separate time hopping communicationschannels C1-C15 is shown.

Channels C1-C5 are defined by a sequence of time slots wherein eachchannel utilizes the same one of the N time slots in each frame. Thus inevery frame, channel C1 uses slot S1, channels C2 uses slot S2, channelC3 uses slot S3, channel C4 uses slot S4 and channel C5 uses slot S5.Channels C6-C15, in contrast, are defined by a series of different onesof said N time slots. For example, looking at channel C6, it can be seenthat it utilizes slot S4 in the first frame, slot S3 in the secondframe, slot S2 in the third frame, slot S1 in the fourth frame and slotS5 in the fifth frame. Looking at channel C15, as another example, itcan be seen that it utilizes slot S5 in frame 1, slot S3 in the secondframe, slot S1 in the third frame, slot S4 in the fourth frame and slotS2 in the fifth frame. Similarly it can be seen that each of channelsC7-C14 do not use the same slot in every time frame.

Overall, channels C1-C15 are distinct from one another. In other words,none of the channels consistently use the same time slots in the sametime frame as any of the other channels. Thus no one channel cancontinuously interfere with another channel. Assume, for example, thatchannel C1 is constantly being used. In the first time frame, channel C1would interfere with users of-channels C9 and C12. In the second timeframe, channel C1 would interfere with users channels C7 and C13.Similarly, in the third through fifth frames, channel C1 would interfererespectively with the following pairs of channels C10 and C14, C8 andC15, and, C6 and C11. Thus instead of continuously interfering with anyone or two channels, the interference caused by channel C1 isdistributed among channels C6-C15 in the five time frames shown.

The five frames of slot sequences for channels C1-C15 illustrated inFIG. 6 is merely illustrative of the benefits that can be obtained in atime hopping radio communications system which utilizes N time slots todefine greater than N distinct time hopping radio communication channelsin accordance with the present invention. One characteristic of thesesequences of time slots is that they are distinct from one another. Inorder for there to be more channels than time slots, at least some ofthe sequences must utilize different ones of said N time slots. Itshould be noted that the sequences shown in the progression of fiveframes in FIG. 6 could either be repeated or could be continued inaccordance with a practically infinite variety of sequences. As long asthe sequence of time slots which defines each channel is substantiallydistinct from the others, then there will be some interferenceaveraging. The degree of the interference averaging will depend uponsequence of slots which define each channel and their relationship toone another.

Because the time hopping radio communications system described hereinaverages interference between channels, it is not a blocking system.Thus when additional channels beyond the number of time slots in thesystem are added to the system, they do not block any one channel of thesystem. Instead, the interference caused by the extra channels isdistributed among all channels. This averaged or distributedinterference can then be cleaned up by known techniques of errorcorrection coding.

In a system as set forth above, the transmitter and receiver followidentical predetermined slot selection sequences. They can do thisaccording to a predetermined sequence. Alternatively they can selectslots in accordance with numbers generated by identical pseudorandomnumber generators. Other methods of synchronizing the transmitter andreceiver will be readily apparent to those skilled in the art.

Like the frequency hopping system previously discussed, the interferenceaveraging time hopping systems described herein are particularly usefulto define communication channels for adjacent or nearby communicationsites such as those shown in FIGS. 1 and 2. A single carrier frequency,supporting a series of frames which are further divided into N timeslots, can be used in each of the adjacent communications sites toprovide greater than N time hopping radio communication channels. Thisresult is attained by defining a first set of orthogonal time hoppingcommunications channels associated with the first of the communicationssites 102 such that no two of the channels in the first set employ thesame one of the N time slots in the same time frame, i.e. concurrently.A second set of orthogonal time hopping communications channelsassociated with the second of the adjacent communications sites 104 canbe defined such that no two of the channels in the second set ofchannels use the same one of the N time slots in a given time frame.Further, a third set of the orthogonal time hopping communicationschannels associated with the third of the adjacent communications sites106 is defined such that no two of the channels in the third set employthe same one of the N time slots at the same time.

Thus, each of the three sets of channels is self orthogonal. In apreferred embodiment of the invention, at least one set of theorthogonal time hopping radio communication channels is defined so thatit is minimally cross-correlated with one or more of the otherorthogonal sets.

It will be apparent that the time hopping embodiment of the presentinvention can also be used in connection with a cellular system such asthat shown in FIG. 2 and in connection with the micro site showntherein. Moreover, the cellular sites may be subdivided into sectors aspreviously discussed with respect to the frequency-hopping embodiment.

FIG. 7 depicts the construction of communications channels fromsequences of time slots. In particular, time hopping communicationschannels for use in three nearby or adjacent communication sites, suchas sectors or cells, are defined in accordance with the techniqueillustrated in FIG. 7. Channels C1-C5 are assigned to a first sitewhile, while channels C6-C10 and C11-C15 are assigned to the second andthird sites respectively. Each channel is assigned a unique series oftime slot selection sequences. Each set of five channels for each of thesites is orthogonal. In other words, no channels for a given site caninterfere with other channels within that given site. Orthogonalchannels can be defined by identical patterns of slot selectionsequences which are shifted framewise with respect to each other. Thuseach set of sequences for channels C1-C5 follow identical patterns butare each shifted from each other, frame wise, such that the sequencesare mutually orthogonal. Thus even though these sequences followidentical patterns, because they are shifted frame wise relative to eachother, they are also distinct from one another.

As illustrated in FIG. 7, multiple communication channels, using thesame carrier frequency subdivided into frames of N time slots each, areattained by allocating the time slots to each communications channels ina preselected sequence. Each channel is defined by a series of differentones of said N time slots. In other words, none of the channels shown inFIG. 7 use the same time slot in each frame. These unique slot selectionsequences are selected so that they are orthogonal to one another ineach site. In other words, the cross-correlation between the sequencesfor a given site or sector is zero.

The slot selection sequences are selected such that channels in eachsite are assigned mutually orthogonal sequences, and intra-sitecorrelation of the slot selection sequences is zero. In the preferredembodiment of this aspect of the invention, in any given time frame, aslot associated in one site, can only be interfered with by slotsassociated with one channel in each of the other sites.

With proper selection of system parameters, the time hopping radiocommunication system described herein offers unique advantages. The usercapacity is enhanced by the intrinsic interference averaging afforded bythe system. Moreover, due to the orthogonal operation described herein,interference from co-users of a user's site is eliminated. Whenimplemented in a cellular configuration, as illustrated in FIG. 2, thesystem described herein also yields a one site reuse pattern.

As with the previously discussed frequency hopping channels, the secondand third sets of the time hopping radio communications channels can bedecimated transformations of the first set and the same decimationtransformation may be utilized.

Thus referring back again to FIG. 4, and interpreting it to representselections of time slots, each of the time hopping radio communicationschannels in the first set can be defined by a unique sequence of thetime slots and the decimating transformation is performed on each of thechannels in the first set by selecting time slots from each of the firstset in their sequential order, skipping a first decimation number oftime slots in the sequence, and repeating this process on the remainingtime slots in the sequence of each channel in their remaining order,until all of the time slots in each channel are used to define a secondset of time hopping channels which are minimally cross correlated withrespect to the first set of time hopping radio communications channels.

Similarly, a second decimating transformation may be performed on eachof the time hopping radio communications channels in the first set byselecting time slots from each of the channels in the first set in theirsequential order, skipping a second decimation number of time slots inthe sequence, and repeating this process on the remaining time slots inthe sequence of each channel in their remaining order, until all of thetime slots in each channel of the first set are used to define a thirdset of time hopping channels which are minimally cross correlated withrespect to the first and second sets of time hopping radio communicationchannels.

In accordance with the invention, the first and second decimationnumbers are different and each is less than the minimum factor of N,where the minimum factor of a number is the smallest number, greaterthan one, that can be divided into the number with a remainder of zero.

As previously illustrated with respect to the frequency hoppingembodiment, suppose that Channel 1 of a first set of orthogonal timehopping radio communications channels associated with a firstcommunications site is defined by the following time slot selectionsequence:

    S1 S2 S3 S4 S5 S1 S2 S3 S4 S5 S1 S2 S3 S4 S5 . . .         (Set 1, Ch. 1)

Decimation is executed on Channel 1 of the first set by selecting timeslots from the above-listed time slot selection sequence in sequentialorder, skipping a first number of time slots in the sequence. Thisnumber is referred to herein as the "decimation number" or "decimationfactor." For purposes of this example, suppose a decimation factor of 3.Decimating the above sequence with a decimation factor of 3 yields thefollowing time slot selection sequence:

    S4 S3 S2 S1 S5 S4 S3 S2 S1 S5 S4 S3 S2 S1 S5 . . .         (Set 2, Ch.6)

In accordance with the invention, this sequence is used to defineChannel 6, which is the first channel of a second set of channels. Thesecond set of channels is, of course, associated with a secondcommunications site.

Time slot selection sequences for remaining channels C7-C10 of Set 2 areconstructed similarly, by decimating the sequences for each of thechannels of Set 1 by the same decimation factor of 3.

Then, to generate the time slot selection sequence defining Channel C11,the first channel of the third set of channels--the third set beingassociated with the third communications site--the Set 1, Channel 1sequence is decimated by a different decimation number, selecting timeslots from the Set 1, Channel C1 sequence in their sequential order,skipping the second decimation number of time slots in the sequence. Inparticular, the following sequence can be decimated, for example, by thedecimation factor of 2, such that the decimation of the sequence

    ______________________________________                                        S1 S2 S3 S4 S5 S1 S2 S3 S4 S5 S1 S2 S3 S4 S5 . . .                                                     (Set 1, Ch. 1)                                       yields                                                                        S3 S1 S4 S2 S5 S3 S1 S4 S2 S5 S3 S1 S4 S2 S5 . . .                                                     (Set 3, Ch. 11)                                      ______________________________________                                    

This process is repeated on the remaining time slots in the sequence ofeach channel in their remaining order until all of the time slots ineach channel are used to define a third set of time hoppingcommunications channels which are minimally cross correlated withrespect to the first and second sets.

It will be appreciated that the above-described operations are analogousto those described with respect to defining frequency hopping channelsand that they provide sets of sequences having minimumcross-correlation, and support communications sites or sectors in whichno two of channels in a given site employ the same one of the allocatedtime slots in a given time frame. The resulting time divisionmultiplexed system is thus characterized by orthogonal operation withina given site or sector, and minimum sector-to-sector cross-correlation.Like the frequency hopping system, this system has the property that thenumber of available adjacent communications sites is one less than theminimum factor of the number of time slots per time frame. Once again,the minimum factor of a number is the smallest number, greater than one,that can be divided into the number with a remainder of zero.

It will be understood that changes may be made in the above constructionand in the foregoing sequences of operation without departing from thescope of the invention. It is accordingly intended that all mattercontained in the above description or shown in the accompanying drawingsbe interpreted as illustrative rather than in a limiting sense. It isalso noted that many of the features of the invention described withrespect to the frequency hopping embodiment of the invention may beapplied to the time hopping embodiment. Further, the present inventioncontemplates systems wherein hybrid hopping, i.e. both frequency andtime hopping, is utilized.

Having described the invention, what is claimed as new and secured byLetters Patent is:
 1. A method of providing hopping communicationchannels in a hopping multiple access communications system in which aset of R communications resources are used to provide said hoppingcommunications channels, said method including:defining a plurality ofadjacent communications sites associated with said communicationssystem; using said R communication resources to define a first set ofhopping communications channels associated with a first of saidplurality of adjacent communications sites in which no two of saidchannels in said first set of hopping communications channels employ thesame one of said R communications resources at the same time; and usingsaid R communications resources to define a second set of hoppingcommunications channels associated with a second of said plurality ofadjacent communications sites in which no two of said channels in saidsecond set of hopping communications channels employ the same one ofsaid R communications resources at the same time, said second set ofhopping communications channels being a first decimated transformationof each of said hopping communications channels in said first set. 2.The method of claim 1 further comprising:using said R communicationsresources to define a third set of hopping communications channelsassociated with a third of said plurality of adjacent communicationssites in which no two of said channels in said third set of hoppingcommunications channels employ the same one of said R communicationsresources at the same time, said third set of said hoppingcommunications channels being a second decimated transformation of eachof said hopping communications channels in said first set.
 3. Themultiple access communications system as defined in claim 1 in whicheach of said hopping communications channels in said first set isdefined by a unique sequence of said R communications resources and saidfirst decimated transformation is performed on each of said hoppingcommunications channels in said first set by selecting one of said Rcommunications resources from each of said hopping communicationschannels in said first set in their sequential order skipping a firstdecimation number of communications resources in said sequence andrepeating this process on the remaining communications resources in saidsequence of each channel in their remaining order until all of saidcommunications resources in each channel are used to define said secondset of frequency hopping communications channels.
 4. The multiple accesscommunications system as defined in claim 2 in which each of saidhopping communications channels in said first set is defined by a uniquesequence of said R communications resources and said first decimatedtransformation is performed on each of said hopping communicationschannels in said first set by selecting communications resources fromeach of said hopping communications channels in said first set in theirsequential order skipping a first decimation number of communicationsresources in said sequence and repeating this process on the remainingcommunications resources in said sequence of each channel in theirremaining order until all of said communications resources resources ineach channel are used to define said second set of frequency hoppingcommunications channels; andsaid second decimated transformation isperformed on each of said hopping communications channels in said firstset by selecting communications resources from each of said hoppingcommunications channels in said first set in their sequential orderskipping a second decimation number of communications resources in saidsequence and repeating this process on the remaining communicationsresources in said sequence of each channel in their remaining orderuntil all of said communications resources in each channel are used todefine said third set of frequency hopping communications channels. 5.In a hopping multiple access communications system in which a set of Rcommunications resources are used to provide hopping communicationschannels in said system, said system including:a plurality of adjacentcommunications sites associated with said communications system; and acentral base communications station which uses said R communicationsresources for defining a first set of hopping communications channelsassociated with a first of said plurality of adjacent communicationssites in which no two of said channels in said first set of hoppingcommunications channels employ the same one of said R communicationsresources at the same time; and which also uses said R communicationsresources for defining a second set of hopping communications channelsassociated with a second of said plurality of adjacent communicationssites in which no two of said channels in said second set of hoppingcommunications channels employ the same one of said R communicationsresources at the same time, wherein said second set of hoppingcommunications channels is further defined as a first decimatedtransformation of each of said hopping communications channels in saidfirst set.
 6. In a hopping multiple access communications system asdefinded in claim 5 in which said central base communications stationalso uses said R communications resources for defining a third set ofhopping communications channels associated with a third of saidplurality of adjacent communications sites in which no two of saidchannels in said third set of hopping communications channels employ thesame one of said R communications resources at the same time, whereinsaid third set of hopping communications channels is further defined asa second decimated transformation of each of said hopping communicationschannels in said first set.
 7. In the multiple access hoppingcommunications system as defined in claim 5, said system alsoincluding:means for selectively encoding digital information signals onones of said hopping communications channels such that there is aredundant relationship between channel bits; error correcting means forsetting said relationship; and decoder means for utilizing saidrelationship for error correction.
 8. In the multiple access hoppingcommunications system as defined in claim 5 also including:decodingmeans for decoding said digital information signals which includes theuse of soft decision making and side information.
 9. In the multipleaccess hopping communications system as defined in claim 8 said systemfurther comprising:voice activity detection means for measuring signalactivity levels representative of subscriber voice activity on anassigned channel to generate a channel voice activity signal; and meansresponsive to said channel voice activity signal for selectivelyallocating channels to subscriber.
 10. In the multiple access hoppingcommunications system as defined in claims 5 said system furthercomprising:means for performing a conference call between a plurality ofsubscribers in a site by causing each of said subscribers to employ thesame channel.